Microstructural characterization for phase change and damage of cement-based materials using ultrasonic waves

Abstract

Investigation of microstructure is essential procedure for evaluation of cement-based materials because its evolution and inhomogeneities can significantly affect to the mechanical properties and strength of the concrete structure. This dissertation attempts to evaluate the microstructure in cement-based materials using proposed nondestructive ultrasonic methods. The focus of this dissertation is the characterization of microstructure according to the diverse phase of concrete, such as early-age, normal, and damaged condition. The first study devoted to characterize a microstructural evolution of early-age cement paste using a diffuse ultrasound. Experimental measurement of diffuse ultrasound on P-wave was performed using laser Doppler vibrometer. Change of incoherent component was analyzed to investigate the hydration process of cement paste, where combined time was suggested as useful indicator for setting. The second study was carried out characterization of pore dispersion in hardened cement paste based on the measurement of through-transmitted wave attenuation. Wave attenuation is disturbed by the inclusion of air voids. Thus quantified parameters for characterizations were obtained from comparison between measurement of wave attenuation and theoretical model based on scattering theory, and represent air voids contents, such as volume fraction and size distribution of entrained voids. Application example was discussed with respect to the measurement of permeable pore space, and an optical micrographic analysis was also conducted. The third study focused on the characterization of microcracks due to thermal damage of concrete. Thermally damaged concrete was visualized in a multiscale, and it was characterized as contact-type defects using impact-modulation technique based on a nonlinear ultrasonic method. The measured nonlinearity parameters were evaluated by comparison with measurement of permeable pores. The experimental study was also performed to evalu...